eugene, you can use that as a livewell, bait tank, fish box, beer, pepsi, or whatever you want. For the livewell/bait tank, it looks like your fittings are already in place. One should be for drawing water in from below the water line and another for the drain which should go to a through hull just above the water line. The third? Beats me? Plug it up LOL. Seriously, I'd wait to see what the pro's have to say.
P.S. I wouldn't put fillets in there. I accidently left a cooler full of flounder fillets on my boat and got stopped. That is a no-no and they will write you up in a heart beat. Trust me, they took all my fish that me and my dad worked for hours catching and cleaning. The law is meant to keep people from keeping shorts or more than their limit - niether of which we did and the fillets were obviously not from shorts or over the limit. They guy even admitted that!! He just wanted some fish. Plus it cost me $150 which I had to pay ON THE SPOT! It was a real cheap shot that he took at us knowing we didn't mean to do that.